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THE AMERICAN NATURALIST [Vol. LIV 



2. Role of Moistu re — During all of the experiments 

 performed at Syracuse, it is possible that moisture, evap- 

 orating from the pool and diffusing through the atmos- 

 phere, served as an additional stimulus in effecting a 

 positive response from the gerrids. Such a possibility is 

 more feasible in this connection than was true in the case 

 at White Heath, for the area of the water surface where 

 the experiments were carried on at Syracuse is very much 

 larger than the area of the water surface at the place in 

 the brook where the experiments were conducted at White 

 Heath. 



VIII. Summary and Conclusions 

 This paper treats of certain habitat responses of the 

 large water- strider, Gerris remigis Say. The work was 

 done partly near Urbana, Illinois, and partly near Syra- 

 cuse, New York. Observations were made of the re- 

 sponses of the water-striders trapped in stream pools, 

 during a period of severe drought, for the purpose of 

 discovering what became of them after the pools dried 

 up. These gerrids, being mainly apterous forms, were 

 unable to migrate by flight. Experiments, related to the 

 habitat responses, were performed for the purpose of 

 finding out whether water-striders were able to reach 

 their habitat, a brook of moderate size after having been 

 removed from it and placed on the ground certain dis- 

 tances away. 



In the late summer, during a severe drought, with a 

 temperature from 90° to 100° F., water-striders, Gerris 

 remigis, frequently were found on stream pools, con- 

 nected by small riffles, at White Heath, near Urbana. As 

 food became scarce or when a scum formed on the surface 

 of some of the pools, the gerrids migrated, by way of the 

 riffles, to other pools that were free from scum. As the 

 drought progressed, the water-striders were congregated 

 on the few pools that remained. Often the scum, a bac- 

 terial growth, killed large numbers of the gerrids. 



These stream pools were studied and the responses of 

 the water-striders were observed after the pools dried up. 



